Demodulation apparatus and method in a communication system employing 8-ary PSK modulation

ABSTRACT

An 8-ary PSK demodulation apparatus for receiving an input signal R k (X k , Y k ) comprised of a k th  quadrature-phase component Y k  and a k th  in-phase component X k , and for generating soft decision values Λ(s k,0 ), Λ(s k,1 ), and Λ(s k,2 ) for the input signal R k (X k , Y k ) by a soft decision means. A calculator calculates Z k  by subtracting a level |Y k | of the quadrature-phase signal component Y k  from a level |X k | of the in-phase signal component X k , and outputs the Z k  as a first soft decision value. A first selector selects the Z k  or reverse −Z k , according to an MSB of the quadrature-phase signal component Y k . A second selector selects the Z k  or the reverse −Z k  according to an MSB of the in-phase signal component X k . A third selector selects an output of the second selector or a value “0” according to an MSB of the Z k . A first adder adds {square root}{square root over (2)}Y k  to an output of the third selector, and outputs the result value as a third soft decision value. A fourth selector selects an output of the second selector or a value “0” according to the MSB of the Z k . A second adder adds {square root}{square root over (2)}X k  to an output of the fourth selector, and outputs the result value as a second soft decision value.

[0001] This application claims priority to an application entitled “Method and Apparatus for Calculating Soft-Decision Value for Multi-Level Modulation” filed in the Korean Industrial Property Office on Aug. 14, 2001 and assigned Serial No. 2001-48921, the contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to a demodulation apparatus and method in a digital communication system employing multi-level modulation, and in particular, to a demodulation apparatus and method for calculating soft decision values needed as inputs to a channel decoder in a demodulator for a digital communication system employing 8-ary PSK (Phase Shift Keying) modulation.

[0004] 2. Description of the Related Art

[0005] In a digital communication system employing 8-ary PSK modulation, a kind of multi-level modulations, to increase spectral efficiency, a signal encoded by a channel encoder is transmitted after being modulated. A demodulator then demodulates the signal transmitted and provides the demodulated signal to a channel decoder for decoding. The channel decoder performs soft decision decoding in order to correct errors. To do so, the demodulator must have a mapping algorithm for generating soft decision values (or soft values) corresponding to output bits of the channel encoder from a 2-dimensional signal comprised of an in-phase signal component and a quadrature-phase signal component.

[0006] The mapping algorithm is classified into a simple metric procedure proposed by Nokia and a dual minimum metric procedure proposed by Motorola. Both algorithms calculate LLR (Log Likelihood Ratio) for the output bits and use the calculated LLR as an input soft decision value of the channel decoder.

[0007] The simple metric procedure, a mapping algorithm given by modifying a complex LLR calculation formula into a simple approximate formula, has a simple LLR calculation formula, but LLR distortion caused by the use of the approximate formula leads to performance degradation. The dual minimum metric procedure, a mapping algorithm of calculating LLR with a more accurate approximate formula and using the calculated LLR as an input soft decision value of the channel decoder, can make up for performance degradation of the simple metric procedure to some extent. However, compared with the simple metric procedure, this procedure needs increased calculations, thus causing a considerable increase in hardware complexity.

SUMMARY OF THE INVENTION

[0008] It is, therefore, an object of the present invention to provide an apparatus and method for simplifying obtaining input soft values of a channel decoder, calculated by the dual minimum metric procedure, without a mapping table or complex processing needed to obtain a minimum distance value with a received signal in a demodulator for a digital communication system employing 8-ary PSK modulation.

[0009] It is another object of the present invention to provide an apparatus and method for calculating a soft decision value by a simple conditional formula in a digital communication system employing 8-ary PSK modulation.

[0010] To achieve the above and other objects, there is provided an 8-ary PSK demodulation apparatus for receiving an input signal R_(k)(X_(k), Y_(k)) comprised of a k^(th) quadrature-phase component Y_(k) and a k^(th) in-phase component X_(k), and for generating soft decision values Λ(s_(k,0)), Λ(s_(k,1)), and Λ(s_(k,2)) for the input signal R_(k)(X_(k), Y_(k)) by a soft decision means. The apparatus comprises a calculator for calculating a soft value Z_(k) by subtracting a level |Y_(k)| of the quadrature-phase signal component Y_(k) from a level |X_(k)| of the in-phase signal component X_(k) of the received signal R_(k)(X_(k), Y_(k)), and outputting the Z_(k) as a first soft decision value; a first selector for receiving the Z_(k) from the calculator and a inverted value −Z_(k) of the Z_(k), and selecting the Z_(k) or the −Z_(k) according to a most significant bit (MSB) of the quadrature-phase signal component Y_(k); a second selector for receiving the Z_(k) from the calculator and the −Z_(k), and selecting the Z_(k) or the −Z_(k) according to an MSB of the in-phase signal component X_(k); a third selector for receiving an output of the second selector and a value “0”, and selecting the output of the second selector or the value “0” according to an MSB of the function Z_(k); a first adder for adding a value calculated by multiplying the quadrature-phase signal component Y_(k) by {square root}{square root over (2)} to an output of the third selector, and outputting the result value as a third soft decision value; a fourth selector for receiving an output of the second selector and a value “0”, and selecting the output of the second selector or the value “0” according to the MSB of the function Z_(k); and a second adder for adding a value calculated by multiplying the in-phase signal component X_(k) by {square root}{square root over (2)} to an output of the fourth selector, and outputting the result value as a second soft decision value

[0011] To achieve the above and other objects, there is provided an 8-ary PSK demodulation method for receiving an input signal R_(k)(X_(k), Y_(k)) comprised of a k^(th) quadrature-phase component Y_(k) and a k^(th) in-phase component X_(k), and for generating soft decision values Λ(s_(k,0)), Λ(s_(k,1)), and Λ(s_(k,2)) for the input signal R_(k)(X_(k), Y_(k)) by a soft decision means. The method comprises the steps of: (a) calculating a soft value Z_(k) of a first demodulated symbol by subtracting a level |Y_(k)| of the quadrature-phase signal component Y_(k) from a level |X_(k)| of the in-phase signal component X_(k) of the received signal R_(k)(X_(k), Y_(k)),(b) setting a first variable α to “0” if the soft value Z_(k) has a positive value, setting the first variable α to “−1” if the Z_(k) has a negative value and the quadrature-phase component Y_(k) has a positive value, and setting the first variable α to “1” if the Z_(k) has a negative value and the quadrature-phase component Y_(k) has a negative value; (c) determining a soft value of a third demodulated symbol by calculating {square root}{square root over (2)}Y_(k)+α*Z_(k) using the quadrature-phase component Y_(k), the soft value Z_(k) and the first variable α; (d) setting a second variable β to “0” if the soft value Z_(k) has a negative value, setting the second variable β to “−1” if the Z_(k) has a positive value and the in-phase component X_(k) has a negative value, and setting the second variable β to “1” if the Z_(k) has a positive value and the in-phase component X_(k) has a positive value; (e) determining a soft value of a second demodulated symbol by calculating {square root}{square root over (2)}X_(k)+β*Z_(k) using the in-phase component X_(k), the soft value Z_(k) and the second variable β.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which:

[0013]FIG. 1 illustrates a signal constellation with mapping points according to 8-ary PSK;

[0014]FIG. 2 illustrates a procedure for calculating soft decision values in a digital communication system employing 8-ary PSK according to an embodiment of the present invention;

[0015]FIG. 3 illustrates block diagram of a calculator for determining soft decision values of a demodulated symbol according to an embodiment of the present invention;

[0016]FIG. 4 illustrates a logic circuit of a soft decision value calculator for use in a digital communication system employing 8-ary PSK; and

[0017]FIG. 5 illustrates a signal constellation having mapping points according to the 8-ary PSK, for explanation of calculations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018] A preferred embodiment of the present invention will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail since they would obscure the invention in unnecessary detail.

[0019] The present invention provides a method for calculating multi-dimensional soft decision values needed as inputs to a channel decoder from a 2-dimentional received signal, using the dual minimum metric procedure.

[0020] In a transmitter, a modulator divides an output bit stream of a channel encoder into m-bit signal sequences, and maps the signal sequences to corresponding signal points among M (=2^(m)) signal points according to a Gray coding rule. This can be represented by

[0021] Equation (1)

[0022] $\begin{matrix} {{{s_{k,{m - 1}}{s_{k,{m - 2}}.\quad.\quad.s_{k,0}}}\overset{f}{\rightarrow}I_{k}},Q_{k}} & {{Equation}\quad (1)} \end{matrix}$

[0023] In Equation (1), s_(k,1) (i=0,1, . . . ,m−1) indicates an i^(th) bit in a signal sequence mapped to a k^(th) symbol, and I_(k) and Q_(k) indicate an in-phase (I) signal component and a quadrature-phase (Q) signal component of the k^(th) symbol, respectively. For 8-ary PSK, m=3 and a corresponding signal constellation are illustrated in FIG. 1. As illustrated, the signal constellation includes 8 (=2³) mapping points, each point having a 45° phase difference with the mapping points adjacent thereto.

[0024] As illustrated in FIG. 1, a symbol is mapped to an in-phase signal component I_(k) and a quadrature-phase signal component Q_(k), and transmitted to a receiver through transmission media. Upon receiving the in-phase signal component and the quadrature-phase signal component, the receiver demodulates the received signal components at a symbol demodulator. A received signal corresponding to the transmitted signal comprised of the in-phase signal component I_(k) and the quadrature-phase signal component Q_(k) can be expressed in a complex number by Equation (2) below, taking a transmission gain and noises into consideration.

[0025] Equation (2)

R _(k) ≡X _(k) +jY _(k) =g _(k)(I _(k) +jQ _(k))+(η^(I) _(k) +jη ^(Q) _(k))

[0026] In Equation (2), X_(k) and Y_(k) indicate an in-phase signal component and a quadrature-phase signal component of a 2-dimensional received signal mapped to a k^(th) symbol, respectively. Further, g_(k) is a complex coefficient indicating gains of the transmitter, the transmission media, and the receiver. In addition, η^(I) _(k) and η^(Q) _(k) are Gaussian noises with an average 0 and a divergence σ² _(n), and they are statistically independent of each other.

[0027] The symbol demodulator of the receiver calculates LLR using a received signal R_(k) of Equation (2). The LLR corresponding to an i^(th) bit s_(k;i) (i=0,1, . . . , m−1) in the output sequence of the channel encoder in the transmitter can be calculated by Equation (3), and the calculated LLR is provided to the channel decoder in the receiver as a soft decision value.

[0028] Equation (3) $\begin{matrix} {{{\Lambda \left( S_{k\quad i} \right)} = {K\quad \log \frac{P\quad r\left\{ {{S_{ki} = \left. 0 \middle| X_{k} \right.},Y_{k}} \right\}}{P\quad r\left\{ {{S_{ki} = \left. 1 \middle| X_{k} \right.},Y_{k}} \right\}}}}{{i = 0},1,\ldots \quad,{m - 1}}} & {{Equation}\quad (3)} \end{matrix}$

[0029] In Equation (3), Λ(s_(k,i)) is an LLR or a soft decision value corresponding to s_(k,i), k is a constant, and Pr{A|B} indicates a conditional probability defined as a probability that an event A will occur when an event B occurs. However, Equation (3) is non-linear, thus accompanying relatively many calculations. Therefore, it is necessary to approximate Equation (3), for actual realization. In the case of a Gaussian noise channel with g_(k)=1 in Equation (2), Equation (3) can be written as follows.

[0030] Equation (4) $\begin{matrix} {{\Lambda \left( s_{k,i} \right)} = {K\quad \log \frac{\sum\limits_{z_{k}}{\exp \left\{ \left. {{- 1}/\sigma_{\eta}^{2}} \middle| {R_{k} - {z_{k}\left( {s_{k,i} = 0} \right)}} \right|^{2} \right\}}}{\sum\limits_{z_{k}}{\exp \left\{ \left. {{- 1}/\sigma_{\eta}^{2}} \middle| {R_{k} - {z_{k}\left( {s_{k,i} = 1} \right)}} \right|^{2} \right\}}}}} & {{Equation}\quad (4)} \end{matrix}$

[0031] However, Equation (4) is also non-linear. Therefore, Equation (4) can be approximated by the dual minimum metric procedure proposed by Motorola, as follows.

[0032] Equation (5) $\begin{matrix} \begin{matrix} {{\Lambda \left( x_{k,i} \right)} \approx \quad {K\quad \log \frac{\exp \left\{ \left. {{{- 1}/\sigma_{\eta}^{2}}\min} \middle| {R_{k} - {z_{k}\left( {s_{k,i} = 0} \right)}} \right|^{2} \right\}}{\exp \left\{ \left. {{{- 1}/\sigma_{\eta}^{2}}\min} \middle| {R_{k} - {z_{k}\left( {s_{k,i} = 1} \right)}} \right|^{2} \right\}}}} \\ {= \quad {K^{\prime}\left\lbrack \min \middle| {R_{k} - {z_{k}\left( {s_{k,i} = 1} \right)}} \middle| {}_{2} - \right.}} \\ {\quad \left. \min \middle| {R_{k} - {z_{k}\left( {s_{k,i} = 0} \right)}} \right|^{2}} \end{matrix} & {{Equation}\quad (5)} \end{matrix}$

[0033] In Equation (5), K′=(1/σ² _(n))K , and z_(k)(s_(k,i)=0) and z_(k)(s_(k,i)=1) indicate actual values of I_(k)+jQ_(k) for s_(k,i)=0 and s_(k,i)=1, respectively. In order to calculate Equation (5), it is necessary to determine z_(k)(s_(k,i)=0) and z_(k)(s_(k,i)=1) for minimizing |R_(k)−z_(k)(s_(k,i)=0)|² and |R_(k)−z_(k)(s_(k,i)=1)|² for a 2-dimensional received signal R_(k).

[0034] Taking into consideration n_(k,i) indicating an i^(th) bit value of a reverse mapping sequence for a signal point nearest to R_(k) and {overscore (n)}_(k,i) indicating a negation for n_(k,i) Equation (5) approximated by the dual minimum metric procedure can be rewritten as

[0035] Equation (6) $\begin{matrix} \begin{matrix} {{\Lambda \left( s_{k,i} \right)} = \quad {{K^{\prime}\left( {{2n_{k,i}} - 1} \right)}{\quad\left\lbrack \left| {R_{k} - {z_{k}\left( {s_{k,i} = n_{k,i}} \right)}} \middle| {}_{2} - \right. \right.}}} \\ \left. \quad \left. \min \middle| {R_{k} - {z_{k}\left( {s_{k,i} = {\overset{\_}{n}}_{k,i}} \right)}} \right|^{2} \right\rbrack \end{matrix} & {{Equation}\quad (6)} \end{matrix}$

[0036] That is, Equation (6) can be calculated by determining whether an i^(th) bit value n_(k,i) of a reverse mapping sequence for a signal point at the shortest distance from R_(k) is “0” or “1” and determining the minimum {overscore (n)}_(k,i) value for an i^(th) bit value of the reverse mapping sequence. The value calculated by Equation (6) becomes a soft decision value for the i^(th) bit value of the reverse mapping sequence. As the soft decision value becomes larger in a positive or a negative value, the information provided to a channel decoder becomes more correct.

[0037] The signal point at the shortest distance from R_(k) is determined by ranges of an in-phase signal component value and a quadrature-phase signal component value of R_(k). Therefore, a first term in the brackets of Equation (6) can be written as

[0038] Equation (7)

|R _(k) −z _(k)(s _(k,i) =n _(k,i))|²=(X _(k) −U _(k))²+(Y _(k) −V _(k))²

[0039] In Equation (7), U_(k) and V_(k) denote an in-phase signal component and a quadrature-phase signal component of a signal point mapped by n_(k)={n_(k,m−1), . . . , n_(k,i), . . . , n_(k,1), n_(k,0)} respectively.

[0040] Further, a second term in the brackets of Equation (6) can be written as

[0041] Equation (8)

min|R_(k) −z _(k)(s _(k,i) ={overscore (n)} _(k,i))|²=(X _(k) −U _(k,i))²+(Y _(k) −V _(k,i))²

[0042] In Equation (8), U_(k,i) and V_(k,i) denote an in-phase signal component and a quadrature-phase signal component of a signal point mapped by a reverse mapping sequence m_(k)={M_(k,m−i), . . . , m_(k,i) (={overscore (n)}_(k,i)), . . . , m_(k,i), m_(k,0)} of Z_(k) minimizing |R_(k)−z_(k)(s_(k,i)={overscore (n)}_(k,i))|², respectively. Equation (6) is rewritten as Equation (9) by Equation (7) and Equation (8).

[0043] Equation (9) $\begin{matrix} \begin{matrix} {{\Lambda \left( s_{k,i} \right)} = \quad {{K^{\prime}\left( {{2n_{k,i}} - 1} \right)}\left\lbrack {\left\{ {\left( {X_{k} - U_{k}} \right)^{2} + \left( {Y_{k} - V_{k}} \right)^{2}} \right\} -} \right.}} \\ \left. \quad \left\{ {\left( {X_{k} - U_{k,i}} \right)^{2} + \left( {Y_{k} - V_{k,i}} \right)^{2}} \right\} \right\rbrack \\ {= \quad {{K^{\prime}\left( {{2n_{k,i}} - 1} \right)}\left\lbrack {{\left( {U_{k} + U_{k,i} - {2X_{k}}} \right)\left( {U_{k} - U_{k,i}} \right)} +} \right.}} \\ \left. \quad {\left( {V_{k} + V_{k,i} - {2Y_{k}}} \right)\left( {V_{k} - V_{k,i}} \right)} \right\rbrack \end{matrix} & {{Equation}\quad (9)} \end{matrix}$

[0044] From Equation (9), m soft decision values needed as inputs to a channel decoder supporting m-level modulation can be calculated.

[0045] Herein, a process of calculating input soft decision values to the channel decoder by a demodulator in a data communication system employing 8-ary PSK by Equation (9) will be described.

[0046] First, Table 1 is used to calculate {n_(k,2), n_(k,1), n_(k,0)}, U_(k) and V_(k) from two signal components X_(k) and Y_(k) of an 8-ary PSK-modulated received signal R_(k). Table 1 illustrates {(n_(k,2), n_(k,1), n_(k,0)), U_(k) and V_(k) for the case where a received signal R_(k) appears in each of 8 regions centered on the signal points in FIG. 1. For the sake of convenience, 4 boundary values, i.e., result values at X_(k)=0, Y_(k)=0, Y_(k)=X_(k), Y_(k)=−X_(k) are omitted from Table 1. TABLE 1 Condition Condition of Yk of Y_(k)/X_(k) {n_(k,2), n_(k,1), n_(k,0)} U_(k) V_(k) Y_(k) > 0 Y_(k)/X_(k) > 1 {0, 0, 1}  sin(π/8)  cos(π/8)  0 < Y_(k)/X_(k) < 1 {0, 0, 0}  cos(π/8)  sin(π/8) −1 < Y_(k)/X_(k) < 0 {0, 1, 0} −cos(π/8)  sin(π/8)  Y_(k)/X_(k) < −1 {0, 1, 1} −sin(π/8)  cos(π/8) Y_(k) < 0 Y_(k)/X_(k) > 1 {1, 1, 1} −sin(π/8) −cos(π/8)  0 < Y_(k)/X_(k) < 1 {1, 1, 0} −cos(π/8) −sin(π/8) −1 < Y_(k)/X_(k) < 0 {1, 0, 0}  cos(π/8) −sin(π/8) Y_(k)/X_(k) < −1 {1, 0, 1}  sin(π/8) −cos(π/8)

[0047] Further, Table 2 illustrates a sequence {m_(k,2), m_(k,1), m_(k,0)} minimizing |R_(k)−z_(k)(s_(k,i)={overscore (n)}_(k,i))|² calculated for i (where iε{0, 1, 2}), in terms of a function of {n_(k,2), n_(k,1), n_(k,0)}, and also illustrates in-phase and quadrature-phase signal components U_(k,i) and V_(k,i) of the corresponding z_(k). TABLE 2 i {m_(k,2), m_(k,1), m_(k,0)} U_(k,i) V_(k,i) 2 {{overscore (n)}_(k,2), n_(k,1), 0} U_(k,2) V_(k,2) 1 {n_(k,2), {overscore (n)}_(k,1), 1} U_(k,1) V_(k,1) 0 {n_(k,2), n_(k,1), {overscore (n)}_(k,0)} U_(k,0) V_(k,0)

[0048] Table 3 illustrates V_(k,i) and U_(k,i) corresponding to {m_(k,2), m_(k,1), m_(k,0)} determined from Table 2, for all combinations of {n_(k,2), n_(k,1), n_(k,0)}. TABLE 3 {n_(k,2), n_(k,1), n_(k,0)} U_(k,2) U_(k,1) U_(k,0) V_(k,2) V_(k,1) V_(k,0) {0, 0, 1}  cos(π/8) −sin  cos(π/8) −sin  cos(π/8)  sin(π/8) (π/8) (π/8) {0, 0, 0}  cos(π/8) −sin  sin(π/8) −sin  cos(π/8)  cos(π/8) (π/8) (π/8) {0, 1, 0} −cos(π/8)  sin −sin(π/8) −sin  cos(π/8)  cos(π/8) (π/8) (π/8) {0, 1, 1} −cos(π/8)  sin −cos(π/8) −sin  cos(π/8)  sin(π/8) (π/8) (π/8) {1, 1, 1} −cos(π/8)  sin −cos(π/8)  sin −cos(π/8) −sin(π/8) (π/8) (π/8) {1, 1, 0} −cos(π/8)  sin −sin(π/8)  sin −cos(π/8) −cos(π/8) (π/8) (π/8) {1, 0, 0}  cos(π/8) −sin  sin(π/8)  sin −cos(π/8) −cos(π/8) (π/8) (π/8) {1, 0, 1}  cos(π/8) −sin  cos(π/8)  sin −cos(π/8) −sin(π/8) (π/8) (π/8)

[0049] Table 4 illustrates results given by down-scaling, in a ratio of K′({square root}{square root over ((2+2)})−{square root}{square root over ((2−2)})), soft decision values obtained by substituting V_(k,i) and U_(k,i) of Table 3 into Equation (9), i.e., illustrates the results normalized by K′({square root}{square root over ((2+2)})−{square root}{square root over ((2−2)})). That is, when a received signal R_(k) is applied, an LLR satisfying a corresponding condition can be determined as a soft decision value by Table 4. If the channel decoder used in the system is not a max-logMAP (logarithmic maximum a posteriori) decoder, a process of up-scaling the LLR of Table 4 in a reverse ratio of the down-scale ratio must be added. TABLE 4 Condi- tion of Condition of Y_(k) Y_(k)/X_(k) Λ(s_(k,2)) Λ(s_(k,1)) Λ(s_(k,0)) Y_(k) > 0 Y_(k)/X_(k) > 1 {square root}2Y_(k) + {square root}2X_(k)  X_(k) − Y_(k) (−X_(k) + Y_(k))  0 < Y_(k)/X_(k) < 1 {square root}2Y_(k) {square root}2X_(k) + (X_(k) − Y_(k))  X_(k) − Y_(k) −1 < Y_(k)/X_(k) < 0 {square root}2Y_(k) {square root}2X_(k) + (X_(k) + Y_(k)) −X_(k) − Y_(k) Y_(k)/X_(k) < −1 {square root}2Y_(k) + {square root}2X_(k) −X_(k) − Y_(k) (X_(k) + Y_(k)) Y_(k) < 0 Y_(k)/X_(k) > 1 {square root}2Y_(k) + {square root}2X_(k) −X_(k) + Y_(k) (−X_(k) + Y_(k))  0 < Y_(k)/X_(k) < 1 {square root}2Y_(k) {square root}2X_(k) + (X_(k) − Y_(k)) −X_(k) + Y_(k) −1 < Y_(k)/X_(k) < 0 {square root}2Y_(k) {square root}2X_(k) + (X_(k) + Y_(k))  X_(k) + Y_(k) Y_(k)/X_(k) < −1 {square root}2Y_(k) + {square root}2X_(k)  X_(k) + Y_(k) (X_(k) + Y_(k))

[0050] However, when performing 8-ary PSK soft decision demodulation using Table 4, the demodulator should first perform a condition determining operation, including a dividing operation, on the two components of a received signal. Thereafter, the demodulator selects a formula corresponding to the result of the condition determining operation among the formulas designated according to the conditions, and substitutes the two components of the received signal into the selected formula, thereby calculating soft decision values. To this end, the demodulator requires an operator for performing the dividing operation and a memory for storing different formulas according to the condition.

[0051] In order to exclude the dividing operation and remove the memory, it is necessary to modify condition determining formulas and derive soft decision value calculation formulas that can be commonly applied even to the different conditions. To this end, the condition determining formulas shown in Table 4 can be expressed as shown in Table 5, using a new function Z_(k) defined as |X_(k)|−|Y_(k)|. In Table 5, the dividing operations are excluded and the soft decision values at the 4 boundary values, which were omitted from Table 4 for convenience, are taken into consideration. TABLE 5 Con- Con- Con- dition dition dition of Y_(k) of X_(k) of Z_(k) Λ(s_(k,2)) Λ(s_(k,1)) Λ(s_(k,0)) Y_(k) ≧ 0 X_(k) ≧ 0 Z_(k) ≧ 0 {square root}2Y_(k) {square root}2X_(k) + X_(k) − Y_(k) (X_(k) − Y_(k)) Z_(k) < 0 {square root}2Y_(k) − (X_(k) − Y_(k)) {square root}2X_(k) X_(k) − Y_(k) X_(k) < 0 Z_(k) ≧ 0 {square root}2Y_(k) {square root}2X_(k) − −X_(k) − Y_(k) (−X_(k) − Y_(k)) Z_(k) < 0 {square root}2Y_(k) − {square root}2X_(k) −X_(k) − Y_(k) (−X_(k) − Y_(k)) Y_(k) < 0 X_(k) ≧ 0 Z_(k) ≧ 0 {square root}2Y_(k) {square root}2X_(k) + X_(k) + Y_(k) (X_(k) + Y_(k)) Z_(k) < 0 {square root}2Y_(k) + (X_(k) + Y_(k)) {square root}2X_(k) X_(k) + Y_(k) X_(k) < 0 Z_(k) ≧ 0 {square root}2Y_(k) {square root}2X_(k) − −X_(k) + Y_(k) (−X_(k) + Y_(k)) Z_(k) < 0 {square root}2Y_(k) + {square root}2X_(k) −X_(k) + Y_(k) (−X_(k) + Y_(k))

[0052] In hardware realization, Table 5 can be simplified into Table 6 on condition that a sign of X_(k), Y_(k), and Z_(k) can be expressed by their MSB (Most Significant Bit), or sign bit. In Table 6, MSB(x) denotes an MSB of a given value x. TABLE 6 MSB(Y_(k)) MSB(X_(k)) MSB(Z_(k)) Λ(s_(k,2)) Λ(s_(k,1)) Λ(s_(k,0)) 0 0 0 {square root}2Y_(k) {square root}2X_(k) + Z_(k) Z_(k) 1 {square root}2Y_(k) − Z_(k) {square root}2X_(k) 1 0 {square root}2Y_(k) {square root}2X_(k) − Z_(k) 1 {square root}2Y_(k) − Z_(k) {square root}2X_(k) 1 0 0 {square root}2Y_(k) {square root}2X_(k) + Z_(k) 1 {square root}2Y_(k) + Z_(k) {square root}2X_(k) 1 0 °2Y_(k) {square root}2X_(k) − Z_(k) 1 {square root}2Y_(k) + Z_(k) {square root}2X_(k)

[0053] From Table 6, soft decision values Λ(s_(k,2)), Λ(s_(k,1)), and Λ(s_(k,0)) for each i are expressed as

[0054] Equation (10) $\begin{matrix} {{{{\Lambda \left( s_{k,2} \right)} = {{\sqrt{2}Y_{k}} + {\alpha \cdot Z_{k}}}},{w\quad h\quad e\quad r\quad e}}{\alpha = \left\{ \begin{matrix} {\quad 0} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = 0}} \\ {\quad {- 1}} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = {{1\quad {and}\quad {{MSB}\left( Y_{k} \right)}} = 0}}\quad} \\ {\quad 1} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = {{1\quad {and}\quad {{MSB}\left( Y_{k} \right)}} = 1}}} \end{matrix} \right.}} & {{Equation}\quad (10)} \end{matrix}$

[0055] In Equation (10), a parameter α is 0 for MSB(Z_(k))=0, −1 for MSB(Z_(k))=1 and MSB(Y_(k))=0, and 1 for MSB(Z_(k))=1 and MSB(Y_(k))=1.

[0056] Equation (11) $\begin{matrix} {{{{\Lambda \left( s_{k,1} \right)} = {{\sqrt{2}X_{k}} + {\beta \cdot Z_{k}}}},{w\quad h\quad e\quad r\quad e}}{\beta = \left\{ \begin{matrix} {\quad 0} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = 1}} \\ {\quad {- 1}} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = {{0\quad {and}\quad {{MSB}\left( X_{k} \right)}} = 1}}\quad} \\ {\quad 1} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = {{0\quad {and}\quad {{MSB}\left( X_{k} \right)}} = 0}}} \end{matrix} \right.}} & {{Equation}\quad (11)} \end{matrix}$

[0057] In Equation (11), a parameter β is 0 for MSB(Z_(k))=1, −1 for MSB(Z_(k))=0 and MSB(X_(k))=1, and 1 for MSB(Z_(k))=0 and MSB(X_(k))=0.

[0058] Equation (12)

Λ(s _(k,0))=Z _(k)

[0059] That is, in the digital communication system employing 8-ary PSK, it is possible to actually calculate 3 soft decision values, which are outputs of the demodulator for one received signal or inputs to the channel decoder, using the dual minimum metric procedure of Equation (4), through the simple conditional formulas of Equation (10) to Equation (12). This process is illustrated in FIG. 2.

[0060]FIG. 2 illustrates a procedure for calculating soft decision values in a digital communication system employing 8-ary PSK according to an embodiment of the present invention. Referring to FIG. 2, in step S110, a symbol demodulator calculates Z_(k)=|X_(k)|−|Y_(k)| to define the condition determining formulas shown in Table 4 as a new function. The symbol demodulator analyzes an MSB of the Z_(k) in step S120, in order to determine α and β according to the MSB of the Z_(k) in Equation (1) to Equation (12). As a result of the analysis in step S120, if the MSB of the Z_(k) is “0”, the symbol demodulator proceeds to step S130, and otherwise, proceeds to step S140. In step S130, the symbol demodulator analyzes an MSB of X_(k). As a result of the analysis in step S130, if the MSB of the X_(k) is “1”, the symbol demodulator sets the parameter α to “0” and the parameter β to “−1” in step S150. If the MSB of the X_(k) is “0”, the symbol demodulator sets the parameter α to “0” and the parameter β to “1” in step S160.

[0061] As a result of the analysis in step S120, if the MSB of the Z_(k) is “1”, the symbol demodulator analyzes an MSB of Y_(k) in step S140. As a result of the analysis in step S140, if the MSB of the Y_(k) is “0”, the symbol demodulator sets the parameter α to “−1” and the parameter β to “0” in step S170. If the MSB of the Y_(k) is “1”, the symbol demodulator sets the parameter α to “1” and the parameter β to “0” in step S180. Thereafter, in step S190, the symbol demodulator calculates soft decision values by substituting the parameters α and β determined in the proceeding steps and the received signal into Equation (10) to Equation (12). In this way, the symbol demodulation is performed.

[0062] Summarizing, the process of calculating soft decision values by the dual minimum metric procedure includes a first step of determining the first parameter α and the second parameter β by analyzing a 2-dimensional received signal comprised of an in-phase signal component and a quadrature-phase signal component, and a second step of calculating soft decision values using the 2-dimensional received signal and the first parameter α and the second parameter β determined in the first step. The determined soft decision values of a demodulated symbol are provided to a channel decoder.

[0063]FIG. 3 illustrates a calculator for determining soft decision values of a demodulated symbol according to an embodiment of the present invention. Referring to FIG. 3, the calculator for determining soft decision values by the dual minimum metric procedure in a digital communication system includes a received signal analyzer 10 and a soft decision value output unit 20. The received signal analyzer 10 determines first and second parameters α and β by analyzing a received signal comprised of an in-phase signal component X_(k) and a quadrature-phase signal component Y_(k). The soft decision value output unit 20 then calculates soft decision valuesΛ(s_(k,2)), Λ(s_(k,1)), and Λ(s_(k,0)) required for soft decision decoding, using the received signal and the determined parameters α and β.

[0064] A logic circuit of the calculator for calculating soft decision values in accordance with Equation (10) to Equation (12) is illustrated in FIG. 4. In particular, FIG. 4 illustrates a soft decision value calculator for use in a digital communication system employing 8-ary PSK. The logic circuit of FIG. 4 is included in a demodulator of the digital communication system employing 8-ary PSK, and calculates soft decision values using Equation (10) to Equation (12). Herein, the 2-dimensional received signal R_(k), in-phase signal component X_(k), quadrature-phase signal component Y_(k), variable Z_(k), parameter α, and parameter β are all real numbers, and digital values with a sign bit. In FIG. 4, a calculator 105, an inverter 115, a first MSB extractor 155, a first selector 110, a third MSB extractor 165 and a third selector 120 constitute a structure for determining the first parameter α. Further, the calculator 105, the inverter 115, a second MSB extractor 160, a second selector 135, the third MSB extractor 165 and a fourth selector 140 constitutes a structure for determining the second parameter β.

[0065] Referring to FIG. 4, the calculator 105 calculates Z_(k)=|X_(k)|−|Y_(k)| using an in-phase signal component X_(k) and a quadrature-phase signal component Y_(k) of a 2-dimensional received signal R_(k) mapped to a k^(th) symbol. The inverter 115 inverts a sign of the Z_(k) by calculating the Z_(k) from the calculator 105 by “−1. The first MSB extractor 155 extracts an MSB of the Y_(k) received, and provides the extracted MSB to the first selector 110 as a first select signal. The second MSB extractor 160 extracts an MSB of the X_(k) received, and provides the extracted MSB to the second selector 135 as a second select signal. The third MSB extractor 165 extracts an MSB of the Z_(k) received from the calculator 105, and provides the extracted MSB to the third selector 120 as a third select signal. In addition, the Y_(k) is multiplied by {square root}{square root over (2)} at a first multiplier 130, and the X_(k) is also multiplied by {square root}{square root over (2)} at a second multiplier 150.

[0066] The first selector 110 receives the Z_(k) from the calculator 105 and the “−Z_(k)” from the inverter 115, and selects one of the inputs according to the first select signal from the first MSB extractor 155. The third selector 120 then receives an output of the first selector 110 and a bit “0”, and selects one of the inputs according to the third select signal from the third MSB extractor 165. An output of the third selector 120 is added to an output value {square root}{square root over (2)}Y_(k) of the first multiplier 130 by a first adder 125, generating a third soft decision value Λ(s_(k,2)) of the received signal R_(k) mapped to a k^(th) symbol.

[0067] In addition, the second selector 135 receives the Z_(k) from the calculator 105 and the “−Z_(k)” from the inverter 115, and selects one of the inputs according to the second select signal from the second MSB extractor 160. The fourth selector 140 then receives an output of the second selector 135 and a bit “0”, and selects one of the inputs according to the third select signal from the third MSB extractor 165. An output of the fourth selector 140 is added to an output value {square root}{square root over (2)}X_(k) of the second multiplier 150 by a second adder 145, thus generating a second soft decision value Λ(s_(k,1)) of the received signal R_(k) mapped to the k^(th) symbol.

[0068] Meanwhile, the Z_(k) output from the calculator 105 becomes a first soft decision value Λ(s_(k,0)) of the received signal R_(k) mapped to the k^(th) symbol.

[0069] According to the foregoing description, a conventional soft decision value calculator using the dual minimum metric procedure realized by Equation (5) needs ten or more squaring operations and comparison operations. However, the novel calculator of FIG. 4 realized using Equation (10) to Equation (12) is comprised of 3 adders, 3 multipliers, and 4 multiplexers, contributing to a remarkable reduction in operation time and complexity of the calculator. Table 7 below illustrates a comparison made between Equation (5) and Equations (10) to (12) in terms of the type and number of operations, for iε{0, 1, 2}. TABLE 7 Equation (4) Equations (10) to (12) Operation No of Operations Operation No of Operations Addition 3 × 8 + 3 = 27 Addition 3 Squaring 2 × 8 = 16 Multiplication 3 Comparison 3 × 2 × 3 = 18 Multiplexing 4

[0070] In sum, the present invention derives Table 6 to Table 11 from Equation (6) to Equation (8) and the process of Table 1 to Table 5, in order to reduce a time delay and complexity, which may occur when Equation (4), the known dual minimum metric procedure, or Equation (5), obtained by simplifying the dual minimum metric procedure, is actually realized using the 16-ary QAM. Further, the present invention provides Equation (9) and Equation (10), new formulas used to realize the dual minimum metric-procedure in the 16-ary QAM. In addition, the present invention provides a hardware device realized based on Equation (9) and Equation (10).

[0071] Now, a comparison will be made between a conventional method of calculating a soft decision value Λ(s_(k,2)) using Equation (5) and a new method of calculating the soft decision value Λ(s_(k,2)) using Equation (10). FIG. 5 illustrates a signal constellation having mapping points according to the 8-ary PSK, for explanation of calculations. Referring to FIG. 5, a 2-dimensional received signal R_(k) comprised of an in-phase signal component X_(k) and a quadrature-phase signal component Y_(k) has a coordinate value represented by “x.” Herein, it will be assumed that X_(k)=−0.6 and Y_(k)=−0.1.

[0072] First, a conventional process of calculating a soft decision value Λ(s_(k,2)) using Equation (5) will be described.

[0073] The square of each distance between a received signal R_(k) and 4 mapping points with s_(k,2)=1 (i.e., 4 mapping points under an x-axis in FIG. 5) is first calculated to determine the shortest distance.

[0074] The square of a distance from a mapping point “110”={−0.6−cos(9π/8)}²+{−0.1−sin(9π/8)}²=0.185

[0075] The square of a distance from a mapping point “111”={−0.6−cos(11π/8)}²+{−0.1−sin(11π/8)}²=0.726

[0076] The square of a distance from a mapping point “101”={−0.6−cos(13π/8)}²+{−0.1−sin(13π/8)}²=1.644

[0077] The square of a distance from a mapping point “100”={−0.6−cos(15π/8)}²+{−0.1−sin(15π/8)}²=2.402

[0078] Therefore, the minimum value (or the shortest distance from the received signal R_(k)) |R_(k)−z_(k)(s_(k,2)−1)|² is 0.185.

[0079] Then, the square of each distance between the received signal R_(k) and 4 mapping points with s_(k,2)=0 (i.e., 4 mapping points over the x-axis in FIG. 5) is calculated to determine the shortest distance.

[0080] The square of a distance from a mapping point “000”={−0.6−cos(π/8)}²+{−0.1−sin(π/8)}²=2.555

[0081] The square of a distance from a mapping point “001”={−0.6−cos(3π/8)}²+{−0.1−sin(3π/8)}²=2.014

[0082] The square of a distance from a mapping point “011”={−0.6−cos(5π/8)}²+{−0.1−sin(5π/8)}²=1.096

[0083] The square of a distance from a mapping point “010”={−0.6−cos(7π/8)}²+{−0.1−sin(7π/8)}²=0.338

[0084] Therefore, the minimum distance of |R_(k)−z_(k)(s_(k,2)=1)|² is 0.338.

[0085] If the above results are substituted into Equation (5), then the soft decision value becomes

Λ(s _(k,2))=K′[min|R_(k) −z _(k)(s _(k,2)=1)|²−min|R _(k) −z _(k)(s _(k,2)=0)|²]

=K′×(0.185−9.338)

=−0.153×K′

[0086] Next, a new process of calculating a soft decision value Λ(s_(k,2)) using Equation (10) will be described.

[0087] Z_(k) and α are first calculated.

Z _(k) =|X _(k) −|Y _(k)|=|−0.6|−|−0.1|=0.5

[0088] From this, since Z_(k)≧0, i.e., MSB(Z_(k))=0, α=0.

[0089] If the above results are substituted into Equation (10), then the soft decision value becomes

Λ(s _(k,2))={square root}{square root over (2)}Y _(k) +α·Z _(k)={square root}{square root over (2)}×(−0.1)+0×0.5=−0.141

[0090] Here, the reason that the result of Equation (5) is different from the result of Equation (10) is because a soft decision value calculated by Equation (9) was normalized by K′({square root}{square root over ((2+2)})−{square root}{square root over ((2−2)})). In the case of a turbo decoder using a max-logMAP core (currently, both L3QS and 1×TREME use max-logMAP core), normalizing all LLR values (or soft values) using the same coefficient never affects performance.

[0091] If a coefficient is actually multiplied to calculate a non-normalized value, then

−0.141×K′{square root}{square root over ((2+)}{square root over (2))}−{square root}{square root over ((2−)}{square root over (2))})=−0.141×1.082×K′=−0.153×K′

[0092] It is noted that the calculated non-normalized value is identical to the result of Equation (5).

[0093] Summarizing, in order to reduce a time delay and complexity caused by the use of the dual minimum metric procedure of Equation (5), the present invention draws the mapping tables of Table 4 to Table 6 through the process of Equation (6) to Equation (9) and Table 1 to Table 3. Further, the present invention substitutes the mapping tables into Equation (10) to Equation (12), the dual minimum metric procedure realizing formulas. In addition, the present invention provides a logic circuit of a calculator for calculating 8-ary PSK soft decision values, realized by Equation (10) to Equation (12).

[0094] As described above, in deriving a soft decision value needed as an input to a channel decoder by the dual minimum metric procedure, the novel demodulator for a digital communication system employing 8-ary PSK modulation enables simple and rapid calculations, contributing to a remarkable reduction in an operation time and complexity of the demodulator that calculates soft decision values.

[0095] While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 

What is claimed is:
 1. An 8-ary PSK (Phase Shift Keying) demodulation apparatus for receiving an input signal R_(k)(X_(k), Y_(k)) comprised of a k^(th) quadrature-phase component Y_(k) and a k^(th) in-phase component X_(k), and for generating soft decision values Λ(s_(k,0)), Λ(s_(k,1)), and Λ(s_(k,2)) for the input signal R_(k)(X_(k), Y_(k)) by a soft decision means, comprising: a received signal analyzer for calculating a function Z_(k) of the input signal R_(k)(X_(k), Y_(k)) according to an equation Z_(k)=|X_(k)|−|Y_(k)|, and determining a first parameter α and a second parameter β by the input signal; and a soft decision value output unit for calculating the soft decision values for the input signal R_(k)(X_(k), Y_(k)), using the first parameter α, the second parameter β, and the received signal R_(k)(X_(k), Y_(k)), according to ${{\Lambda \left( s_{k,2} \right)} = {{\sqrt{2}Y_{k}} + {\alpha \cdot Z_{k}}}},{{w\quad h\quad e\quad r\quad e\quad \alpha} = \left\{ {{{\begin{matrix} {\quad 0} & {\quad {{i\quad f\quad Z_{k}} \geq 0}} \\ {\quad {- 1}} & {\quad {{i\quad f\quad Z_{k}} < {0\quad {and}\quad Y_{k}} \geq 0}\quad} \\ {\quad 1} & {\quad {{i\quad f\quad Z_{k}} < {0\quad {and}\quad Y_{k}} < 0}} \end{matrix}{\Lambda \left( s_{k,2} \right)}} = {{\sqrt{2}X_{k}} + {\beta \cdot Z_{k}}}},{{w\quad h\quad e\quad r\quad e\quad \beta} = \left\{ \begin{matrix} {\quad 0} & {\quad {{i\quad f\quad Z_{k}} < 0}} \\ {\quad {- 1}} & {\quad {{i\quad f\quad Z_{k}} \geq {0\quad {and}\quad X_{k}} < 0}\quad} \\ {\quad 1} & {\quad {{i\quad f\quad Z_{k}} \geq {0\quad {and}\quad X_{k}} \geq 0}} \end{matrix} \right.}} \right.}$

Λ(s _(k,0))=Z _(k) where Λ(s_(k,i)) indicates a soft decision value corresponding to s_(k,i)(i=0,1,2), and s_(k,i) indicates an i^(th) bit in a coded signal sequence mapped to a k^(th) symbol.
 2. An 8-ary PSK (Phase Shift Keying) demodulation method for receiving an input signal R_(k)(X_(k), Y_(k)) comprised of a k^(th) quadrature-phase component Y_(k) and a k^(th) in-phase component X_(k), and for generating soft decision values Λ(s_(k,0)), Λ(s_(k,1)), and Λ(s_(k,2)) for the input signal R_(k)(X_(k), Y_(k)) by a soft decision means, comprising the steps of: calculating a soft value Z_(k) of the input signal R_(k)(X_(k), Y_(k)) according to equation Z_(k)=|X_(k)|−|Y_(k)|, and determining a first parameter α and a second parameter β by the input signal; and calculating the soft decision values for the input signal R_(k)(X_(k), Y_(k)), using the first parameter α, the second parameter β, and the received signal R_(k)(X_(k), Y_(k)), according to ${{\Lambda \left( s_{k,2} \right)} = {{\sqrt{2}Y_{k}} + {\alpha \cdot Z_{k}}}},{{w\quad h\quad e\quad r\quad e\quad \alpha} = \left\{ {{{\begin{matrix} {\quad 0} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = 0}} \\ {\quad {- 1}} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = {{1\quad {and}\quad {{MSB}\left( Y_{k} \right)}} = 0}}\quad} \\ {\quad 1} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = {{1\quad {and}\quad {{MSB}\left( Y_{k} \right)}} = 1}}} \end{matrix}{\Lambda \left( s_{k,1} \right)}} = {{\sqrt{2}X_{k}} + {\beta \cdot Z_{k}}}},{{w\quad h\quad e\quad r\quad e\quad \beta} = \left\{ \begin{matrix} {\quad 0} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = 1}} \\ {\quad {- 1}} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = {{0\quad {and}\quad {{MSB}\left( X_{k} \right)}} = 1}}\quad} \\ {\quad 1} & {\quad {{i\quad f\quad {{MSB}\left( Z_{k} \right)}} = {{0\quad {and}\quad {{MSB}\left( X_{k} \right)}} = 0}}} \end{matrix} \right.}} \right.}$

Λ(s _(k,0))=Z _(k) where Λ(s_(k,i)) indicates a soft decision value corresponding to s_(k,i)(i=0,1,2), and s_(k,i) indicates an i^(th) bit in a coded signal sequence mapped to a k^(th) symbol.
 3. An 8-ary PSK (Phase Shift Keying) demodulation apparatus for receiving an input signal R_(k)(X_(k), Y_(k)) comprised of a k^(th) quadrature-phase component Y_(k) and a k^(th) in-phase component X_(k), and for generating soft decision values Λ(s_(k,0)), Λ(s_(k,1)), and Λ(s_(k,2)) for the input signal R_(k)(X_(k), Y_(k)) by a soft decision means, comprising: a calculator for calculating a soft value Z_(k) by subtracting a level |Y_(k)| of the quadrature-phase signal component Y_(k) from a level |X_(k)| of the in-phase signal component X_(k) of the received signal R_(k)(X_(k), Y_(k)), and outputting the Z_(k) as a first soft decision value; a first selector for receiving the Z_(k) from the calculator and an inverted value −Z_(k) of the Z_(k), and selecting the Z_(k) or the −Z_(k) according to a most significant bit (MSB) of the quadrature-phase signal component Y_(k); a second selector for receiving the Z_(k) from the calculator and the −Z_(k), and selecting the Z_(k) or the −Z_(k) according to an MSB of the in-phase signal component X_(k); a third selector for receiving an output of the second selector and a value “0”, and selecting the output of the second selector or the value “0” according to an MSB of the Z_(k); a first adder for adding a value calculated by multiplying the quadrature-phase signal component Y_(k) by {square root}{square root over (2)} to an output of the third selector, and outputting a result value as a third soft decision value; a fourth selector for receiving an output of the second selector and a value “0”, and selecting the output of the second selector or the value “0” according to the MSB of the Z_(k); and a second adder for adding a value calculated by multiplying the in-phase signal component X_(k) by {square root}{square root over (2)} to an output of the fourth selector, and outputting a result value as a second soft decision value.
 4. An 8-ary PSK (Phase Shift Keying) demodulation method for receiving an input signal R_(k)(X_(k), Y_(k)) comprised of a k^(th) quadrature-phase component Y_(k) and a k^(th) in-phase component X_(k), and for generating soft decision values Λ(s_(k,0)), Λ(s_(k,1)), and Λ(s_(k,2)) for the input signal R_(k)(X_(k), Y_(k)) by a soft decision means, comprising the steps of: (a) calculating a soft value Z_(k) of a first demodulated symbol by subtracting a level |Y_(k)| of the quadrature-phase signal component Y_(k) from a level |X_(k)| of the in-phase signal component X_(k) of the received signal R_(k)(X_(k), Y_(k)); (b) setting a first variable α to “0” if the soft value Z_(k) has a positive value, setting the first variable α to “−1” if the Z_(k) has a negative value and the quadrature-phase component Y_(k) has a positive value, and setting the first variable α to “1” if the Z_(k) has a negative value and the quadrature-phase component Y_(k) has a negative value; (c) determining a soft value of a third demodulated symbol by calculating {square root}{square root over (2)}Y_(k)+α*Z_(k) using the quadrature-phase component Y_(k), the soft value Z_(k) and the first variable α; (d) setting a second variable β to “0” if the soft value Z_(k) has a negative value, setting the second variable β to “−1” if the Z_(k) has a positive value and the in-phase component X_(k) has a negative value, and setting the second variable β to “1” if the Z_(k) has a positive value and the in-phase component X_(k) has a positive value; and (e) determining a soft value of a second demodulated symbol by calculating {square root}{square root over (2)}X_(k)+β*Z_(k) using the in-phase component X_(k), the soft value Z_(k) and the second variable β. 